Mutwo.music

Changed
- simplify the structure of `Pitch.PitchEnvelope` and `Pitch.PitchIntervalEnvelope`
- renamed `make_generic_pitch_interval` to `cents_to_pitch_interval`
- renamed `make_generic_pitch` to `frequency_to_pitch`
- don't create on-the-fly classes, but simply use `DirectPitch` and `DirectPitchInterval`

Added
- various standard converters to extract from a simple event musical parameters:
- `SimpleEventToPitchList`
- `SimpleEventToVolume`
- `SimpleEventToPlayingIndicatorCollection`
- `SimpleEventToNotationIndicatorCollection`
- `SimpleEventToGraceNoteSequentialEvent`
- `SimpleEventToAfterGraceNoteSequentialEvent`

Changed
- `LoudnessToAmplitudeConverter` to `LoudnessToAmplitude`
- `RhythmicalStrataToIndispensabilityConverter` to `RhythmicalStrataToIndispensability`
- `TwoPitchesToCommonHarmonicsConverter` to `TwoPitchesToCommonHarmonics`

Changed
- package structure to namespace package to apply refactor of mutwo main package

Changed
- public API and general setup of `converters.symmetrical.playing_indicators` module

Added
- syntactic sugar for setting pitch envelope